1. Field of the Invention
The present invention relates to a flash output control apparatus of a flash device useful when flash light photography is carried out with a camera. More particularly, the present invention relates to such flash output control apparatus for a TTL automatic flash output control type of camera provided with a focal plane shutter.
2. Description of the Prior Art
It is known in the art to carry out flash light photography employing a TTL automatic flash output control system. A camera hitherto proposed for this purpose includes a photo receptor provided within the camera to receive the reflected light from the object being illuminated by the flash light. The photo receptor is so disposed as to receive the light transmitted through a taking lens in the camera. Also, a flash output control circuit is provided to determine the object illumination time depending upon the output from the photo receptor and to terminate the flashing of the flash device at the end of the determined illumination time. The flash output control circuit includes a timer circuit for counting the time necessary for controlling the flash output. The timer circuit is made to start counting the time in relation to the running of the opening blade of the focal plane shutter. In this type of camera, according to the prior art, to start the operation of the timer circuit, there has been used a signal derived from a trigger switch which is actuated in synchronism with the start of running of the opening shutter blade.
However, the use of a trigger switch signal as the start signal for the timer circuit has a problem as will be described hereinafter.
While the timer circuit is made to start operating by the signal of the trigger switch, the timing for starting flashing of the flash device, on the other hand, is given by a synchronous switch. As known to those skilled in the art, this synchronous switch is required to operate in synchronism with the completion of running of the opening shutter blade. Since, as previously noted, the trigger switch from which the start signal for the timer circuit is obtained is actuated in synchronism with the start of running of the opening blade, there is produced a time lag from the start of operation of the timer circuit from the start of flashing so long as the trigger switch is actuated in a timer circuit as conventionally used. This time lag generally amounts to about 10 milliseconds.
During the period of this time lag, the control circuit is active and therefore the quantity of object light during this 10 ms. is integrated into a value of light quantity. 10 ms. later, the flash tube begins flashing and the object light derived from this flash light is integrated by the output control circuit in the same manner as above. Consequently, the integrated value of light quantity during the time lag of 10 ms. is added to the integrated value derived from the real flash light. The result is that the flashing of the flash tube is prematurely terminated by the output control apparatus before the flash output reaches the proper exposure value to the object. Thus, the photograph is taken with underexposure resulting in a poor quality of picture.